316 



to the anatomy, I have only examined M. a., as only this form 

 was at my disposal in alcohol material. 



The structure of the stalk resembles that of M. spicatum 

 (VôcHTiNG 1872, Taf. IV); the central cylinder is surrounded by 

 a broad bark with large air-spaces in a circle, in July a large 

 quantity of starch was found in the rhizomic and orthotropic 

 shoots as also in the roots and leaves in slightly brackish water 

 (Melrakke Heath, North Iceland). 



Both species may have land forms; the leaves of the latter 

 (ScHENCK 1884 a p. 22) have dorsiventral lobes in contrast to 

 the radial lobes of the water form; the epidermis of the 

 leaves has no chlorophyll but has stomata, both conditions 

 being the reverse of the submerged form. The stomata in a 

 land form (M. a) from a muddy lake-beach, Graenavatn in Iceland 

 (M. a.), lie in the longitudinal direction of the leaf section. 



Leaves examined of the above-mentioned form, which 

 resemble those of winter-buds, have no stomata. 



The section of the submerged leaves has a radial structure 

 and resembles that of the stalk; in principal lobes the air- 

 passages are rather well-developed and arranged in circles, but 

 they are much reduced in very thin leaves ; in the lateral lobes 

 they are but few and small and situated irregularly. 



Schilling (1894 p. 326) found mucilage formation in the 

 characteristic leaf-hairs of M. spic, in large quantities especially 

 on the winter-buds, the presence of which he thus also recog- 

 nizes. Other points of the shoot are also enveloped in mucilage 

 from the surrounding leaves. On older leaves only the brown 

 scars from the deciduous trichomes are seen. 



According to Parmentier (1897 p. 138) and Schindler (1904 

 p. 53) M. spicatum is distinguished from most other species 

 by having calcium oxalate crystals only in the stalk, not in the 

 leaves. They also occur in the leaves however of specimens 

 both from Greenland and Iceland, and from Denmark (determined 

 by Dr. A. K. Schindler), though in smaller quantity than in M. 



